It depends on a number of factors, but if you want to decide solely on popularity, then you need to see this: The Most Popular Programming Languages of 2015 | ProgrammableWeb.
At the top you have some consistency, with Java at the top, and C, C++ and C# in the top five. Python, a language that I am becoming fond of and using to replace Perl, is up there at #4.
What is interesting, and not shown, are new and up and coming languages, such as Swift. I expect that to inch into the top 10 in the next few years.
If you want to focus on learning a programming language, and you have no other criteria, pick something from this list.